There is growing demand for equipment to be downsized in line with the development of mobile phones and wireless LAN communications that employ the microwave bands in the electromagnetic spectrum. To downsize terminal equipment, it is necessary to downsize the high frequency devices it employs, such as high frequency filters and resonators.
These high frequency devices are configured with a capacitor and an internal electrode forming a strip line disposed as required in an inner layer of a laminated body composed of dielectric porcelain. The size of a high frequency device using the same resonance mode is in inverse proportion to the square root of the relative permittivity (∈r) of the dielectric material employed. Accordingly, to manufacture a small resonance device, a material with a relatively high relative permittivity is needed. However, the relative permittivity needed varies according to the frequency band used. With equipment adopting ever higher frequencies, though, the demand for high relative permittivity materials is reducing.
Another characteristic required for the dielectric material used for dielectric porcelain is low loss in the high frequency range. In other words, a high Q value or low frequency-temperature characteristic (TCF) is needed. The Q value is inversely related to dielectric loss (tan δ).
On the other hand, attempts are being made to downsize high frequency devices with high performance by adopting a laminated structure of conductor and dielectric porcelain composition. When a conductor is used in a high frequency range, such as the microwave bands, high conductivity is required. Accordingly, Ag, Cu, or their alloys are typically used as an internal electrode. To achieve the laminated structure of conductor and dielectric porcelain composition as described above, it is necessary to simultaneously fire the internal electrode and dielectric porcelain composition. A material which can be densely sintered under firing conditions that avoid dissolution and oxidization of a conductive metal forming the internal electrode is thus needed. More specifically, the dielectric porcelain composition needs to be sintered at a temperature lower than the melting point of the conductive metal used. If Ag is used, the material needs to be sintered at a temperature lower than the melting point of Ag (961° C.). U.S. Pat. No. 5,273,944 proposes a material of the Bi2O3—CaO—Nb2O5 as one example of this type of microwave dielectric porcelain.
However, although conventional Bi2O3—CaO—Nb2O5 porcelain can be sintered at a low temperature of 900° C., allowing firing together with Ag, its mechanical strength is relatively low, which is 140 MPa. Accordingly, when a laminated element is formed, a drop impact test reveals, in particular, that cracking is likely to occur from the terminal electrodes of the laminated element mounted on a printed circuit board.